Comparison of Al3+ and Li+ liquid electrolytes for as deposited and heat treated WO3 thin films for electrochromic applications

Abstract

Al3+ is considered as one of the alternative charge carrier ions to Li+ for WO3 based electrochromic (EC) devices. In this study, WO3 films have been grown by Magnetron Sputtering method and tested in Li+ and Al3+ liquid electrolytes for the comparison of EC performances. In order to see the effects of structure and heat treatment on the EC performance of WO3 films in both liquid electrolytes, films were annealed at 250°C, 350°C and 450°C. As deposited film and films heat treated up to 350°C were amorphous while the film annealed at 450°C crystallized. Surface modifications were observed with annealing, especially for the film annealed at 450 °C. X-ray photoelectron spectroscopic analysis indicated that the binding energies of W 4f did not vary much with annealing. However, noticeable changes were observed after the EC test performed on the WO3 films in trivalent Al3+ compared to the films tested in monovalent Li+ liquid electrolyte. The optic transmittance of the pristine and annealed films was basically the same except the film annealed at 450 °C for which the transmittance decreased by 25%. The Li+ ions have higher insertion and extraction in and out of the annealed films except the film annealed at 450 °C. The optical modulation parameter was highest for the film annealed at 350 °C tested in Al3+ while that of the pristine film was the highest for the films tested in Li+ electrolyte. The diffusion coefficients of Al3+ ions is calculated to be smaller compared to Li+ ions. The WO3 film annealed at 350 °C was found to be the most durable when tested in Al3+ while both pristine and heat-treated films had relatively better long-time durability for the cycle times used.